Ellis came to the Perth Wildcats as a late replacement for Jaron Brown in the 2004/05 season, where he was asked to split time at the 4 and 3 spots. He was the best shooter in the NBL for that season, shooting 61% from the floor and was named by australiabasket.com in the All-NBL 2nd Team, the NBL All-Defensive Team and the NBL All-Newcomers Team. He was named the Round 21 NBL Player of the Week in 2005. In the same season, he also collected the 2005 Gordon Ellis Medal, the Wildcats' highest individual player honour and the team's Best Defensive Player award. He set his NBL-career best 38 points and 15 rebounds vs Crocodiles on November 26, 2004. He finished the 04/05 season with averages of 17.9 point, 9.6 rebounds, 2.7 assists and 2.0 steals per game.

Rosell Ellis led the NBL in rebounding in 2007 by averaging 11.3 rebounds in 27 games. Having played 57 games in the NBL, Ellis has averaged 10.5rpg, 18ppg and 2.1 assists and won the Player of the Week award on two occasions.
Christened as "Mr. Everything", he also clinched the 2007 PBA Fiesta Conference Best Import award in runaway fashion.

He signed with the Townsville Crocodiles in the NBL for the 2007-2008 season. However he injured his pectoral muscle only two games into the season and was consequently forced to undergo season ending surgery. Ellis returned to the Crocodiles in 2011, signing mid-season.